Polymerase Chain Reaction Screening forSalmonellaand EnterohemorrhagicEscherichia colion Beef Products in Processing Establishments

Abstract

Pathogenic Escherichia coli strains on raw or insufficiently cooked foods are of public health concern as serious disease may result from their ingestion. Therefore, many commercial producers of beef products screen for E. coli O157:H7 before shipment. While Salmonella is not considered an adulterant on raw beef products, it is used as an indication of process control. To detect these microorganisms, rapid screening methods are often used to provide results within 8-24 hours after sampling. During 2005-2008, about 971,389 samples from several commercial beef production plants were tested using a rapid screening method based on the polymerase chain reaction to determine if they were presumptively positive for bacterial cells carrying Salmonella or Shiga toxin-producing E. coli-specific genes. Of the product lots sampled (trim, ground beef, and variety meats), 15% were positive for the stx(1) and/or stx(2) (Shiga toxin genes), 9.1% for the eae gene (the attaching and effacing gene [eae] encoding intimin), 3.0% for an rfb gene region (encoding the O157-specific O side chain polysaccharide), and 1.67% for Salmonella by the polymerase chain reaction assay. In general, lots of ground beef showed the lowest frequency of contamination, and variety meats (by-products of carcass evisceration), the highest. Overall, 4.6%, 4.6%, and 0.8% samples were screen-positive for enteropathogenic E. coli, enterohemorrhagic E. coli, and E. coli O157, respectively. Of the E. coli O157-positive samples, 14% were also Salmonella positive. The frequency of screen-positive samples increases during the summer months, probably because of the prevalence of climatic conditions more conducive to microbial growth. The presence of fecal organisms in beef products suggests a failure of sanitary controls during processing and the more prevalent relatives of E. coli O157, Shiga toxin-producing Escherichia coli, enteropathogenic E. coli, and enterohemorrhagic E. coli, serve as more sensitive indicators of contamination than O157 strains alone.